CDMA (Code Division Multiple Access)

Mr. Vishal Pawar

Assistant Professor

MITSOT

Contents

• CDMA Overview

• Spread Spectrum

• CDMA Characteristics

• Advantage/Disadvantage

• Future Trend & Conclusion

CDMA Overview

• Background

– Multiple users per channel based on spreading codes• using direct sequence spread spectrum(DSSS) technique

– Developed by Qualcomm as IS-95

• Special soft handoff capability• “Narrowband CDMA” : 1.25MHz spread bandwidth• “Broadband CDMA” : 10MHz spread bandwidth

CDMA Overview - cont.

Spread Spectrum

• Spread the radio signal over a wide frequency range by modulating– Increasing the frequency of the discrete time signal– Total energy under the power spectrum remains the same

• Directly spreading and despreading the baseband data by a PN(pseudonoise) sequence– PN sequence is identical for both spreading and despreading– Receiver’s correlator distinguishes sender’s signal

• More resistant to multipath effects and more tolerant of interference

Types of Techniques used for SS

• Direct Sequence SS– all users share a huge frequency band. - general method

• Frequency Hopping SS– transmitter changes its carrier frequency often– frequency is constant in each time chip

• Time Hopping SS– message is transmitted only one time slot

• Chirp SS• Hybrid Method SS

Spread Spectrum - cont.

• Processing Gain : – degree of interference rejection– For high values of Gp, information can be transmitted at power

levels below ambient noise– Commercially available SS systems typically implement

processing gains in the 10-100 rage– Purpose is media access via code division rather than lowering

error rates in presence of noise– If Gp is low, more difficult to distinguish between individual.

)log(10s

ssp B

BG =

CDMA Recovery Data

CDMA Characteristics

• Modulation & coding Feature

– data rate - 9.6 kbps

– filtered bandwidth - 1.25MHz

– Coding - Convolution encoding

– Interleaving 20 ms span

CDMA Rake Receiver

• multipath diversity combining to overcome multipath fade

CDMA Voice Compression

• Voice compression technique

– vocoder : encoding scheme applied only to voice signal• Analog voice signal → digital signal• PCM 64kbps data → 8.6, 4.0, 2.0, 0.8kbps• Selective transmission

– channel vocoder

– Linear Prediction Coding

– Vector quantization

CDMA Power Control

– Make all reverse-direction signal in a cell arrive at the base station with same strength

– near-far problem– variable bit rate signal– open loop power control

• Terminal adjusts its transmitter power as a function of the power it measures in the received forward direction signal

– closed loop power control• perform power control adjustment under control of the

base station

CDMA Logical Channel Structure

• Forward Link– Pilot, Sync, Paging, Traffic Channel

– If less than 7 Paging Channels are used, each unused Walsh code becomes a Traffic Channel

Pilot Sync Paging Paging Traffic Traffic Traffic

W=0 W=32 W=1 W=8 W=9 W=10 W=63

CDMA Channel Structure - cont.

– Pilot Channel • MH synchronize to the pilot channel’s SPN code

– Sync Channel• Protocol information, System ID, Network ID, Pilot PN offset,

System Time, Long PN Code, etc.• MH synchronize to Network System

– Paging Channel• Overhead message• Personal Station Directed Message

– Traffic Channel

CDMA Channel Structure - cont.

• Reverse Link

– Access Channel• communicate from MH to BS• response to paging channel message

– Traffic Channel

Access Access Traffic Traffic

Main Advantage of CDMA

• Higher Spectral Efficiency - soft capacity limit• Interference Resistance• Soft Handoff• Improved Privacy• Time Diversity in Multi-path Channel• Longer Battery Life• Cell Design Flexibility• Better Voice Quality - EVRC

Thank you